1. Field of the Invention
The present invention relates to an electrophotography type lithographic form plate, having a high sensitivity and a superior photosensitive characteristic, as a photosensitive material for a semiconductor laser beam which also has excellent printability as a lithographic form plate.
2. Description of the Prior Art
An electrophotographic sensitive material is used for forming an image by a general electrophotographic process, i.e., charging, exposing and developing. Further, a method for using the same, after the image formation, as a form plate for a lithographic printing has been widely used in practice. Such a photosensitive material for an electrophotography type lithographic form plate is one spectrally sensitized to the visible light from a halogen lamp.
Recently, techniques for electronic editing using a semiconductor laser have been developed, whereby a laser printer capable of outputting a document directly from a computer has been practically used. In addition, the range of wavelengths available from a semiconductor laser is being increased from the near infrared region to the visible region. Thereby there have been many developments of electrophotographic sensitive materials for a semiconductor laser beam. Also there has been much research into a printing form plate which is spectrally sensitized to a semiconductor laser beam, so as to form a form plate member directly from a computer.
Electrophotographic sensitive materials using zinc oxide and having a high sensitivity to wavelengths in the near infrared region (700 nm through 1000 nm) are disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 49-5034, 59-78358, 59-22053, 61-275760 and 62-220962.
Such electrophotographic sensitive material using zinc oxide is spectrally sensitized by using polymethinic cyanin dye as a sensitizer. When a cyanin dye contained in a silver salt sensitive agent was originaly used for the spectral sensitization, the sensitization effect was too small to be put into practical use.
There has been a proposal in which, as an improved sensitive agent for zinc oxide, an acid radical (--COOH, --SO.sub.3 H) is introduced into polymethinic cyanin dye, because a substance with acid radical is known to have a high sensitivity. However, while a little increase in sensitization is produced, the sensitivity does not reach a level suitable for practical use. In addition, there are problems of a falling charge amount and a deterioration in the charge retaining factor which are important electric characteristics for a photosensitive material. Also since cyanin dye has a low shelf-life and is affected by light and heat it could not be put to practical use.
For the improvement thereof, the use of additive has been studied, as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 58-42055, 58-58554, 58-59453, 60-26949 and 1-222266. Further an electrophotographic sensitive material having a cyanin dye construction and an improved shelf-life is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-46245.
This improved sensitive material, however, still has problems of falling of charge amount and deterioration in the charge retaining factor, whereby there is none which can be put to practical use from the view point of sensitivity and/or shelf-life.
In the past, even in photosensitive materials developed as an electrophotography type lithographic form plate, there is none which has been further studied regarding desensitization or printing durability.
Tests were conducted on the above improved electrophotographic sensitive material as a lithographic form plate with the following results.
Equal tinting appears all over a non-image area of a printed product when the form plate is processed twice because it becomes having a desensitized, technique used to avoid desensitization of the plate. Even a hand etching treatment having a low desensitization effect may cause tinting to appear on the non-image area of a printed product, and it is impossible to put such material to practical use as a lithographic form plate.
Accordingly, it is an object of the present invention to provide an electrophotography type lithographic form plate having a high sensitivity to semiconductor laser beam with wavelength of 780 nm and 670 nm, superior electrical characteristics, a stability to environmental change, and an improved shelf-life, as well as having an excellent printability as a lithographic form plate and a favorable fat immunity for preventing the generation of tinting in a non-image area of a printed product.